This invention relates to the preparation of thermoplastic containers for the retention of fluids under pressure, such as carbonated beverages and the like. These containers may be prepared from a preform or parison which may be injection molded, followed by blow molding said parison into a suitably shaped container. A typical thermoplastic material is poly (ethylene terephthalate) or PET, although others can be used.
It is particularly desirable to provide means for thickening the bottom wall of these containers in a controllable manner in order to strengthen the bottom wall structure.
The container configuration generally includes a neck portion with a cap-receiving means, a shoulder portion depending therefrom, a side wall or main body portion depending from the shoulder portion and a bottom wall joined to the side wall. In many of these containers the bottom wall has a champagne bottle bottom configuration with an internal, axially inwardly directed, generally or domed or conical part.
The bottom wall of these containers represents a weak part of the container. Also, it is desirable that the bottom shape provide stability for the bottle in an upright, free-standing position.
Thus, beverage under pressure within the container has a tendency to deform the bottom wall, as for example by everting the inwardly directed conical part, thus rendering the bottle unstable and prone to tip.
Many attempts have been made to overcome these problems while at the same time providing a construction which is inexpensive and economical to process. For example, U.S. Pat. No. 3,881,621 provides a ribbed strengthening at the bottom wall; however, this still represents insufficient strengthening. U.S. Pat. No. 4,134,510 provides a plurality of concentric annular strengthening ribs and a plurality of additional intersecting radial ribs in a complex and expensive procedure. U.S. Pat. Nos. 4,620,639, 4,261,948, 4,603,831, and 4,334,627 utilize a plurality of inwardly projecting rigid ribs so that the bottom wall is thicker at the ribbed portion than the remainder of the bottom wall; however, this results in a container having substantial and sharply defined differences in wall thickness with resultant major temperature differences during processing rendering processing difficult.
Copending U.S. patent application Ser. No. 044,469, for Blow Molded Plastic Container and Preform Therefor, By Herbert Strassheimer, Filed May 1, 1987 represents a significant advantage in this art. The plastic preform of said copending application comprises: a neck portion defining an opening; a tubular body portion depending therefrom; and a integral bottom structure depending from the tubular body portion; said preform having an outside wall face and an inside wall face, with at least one of the inside wall face and the outside wall face of the tubular body portion adjacent the bottom structure and extending onto said bottom structure having a plurality of thickened regions, as for example, formed by several flat faces. The blow molded plastic container of said copending application comprises: a neck portion defining an opening; a bottom portion; a tubular body portion interconnecting said neck and bottom portions, said bottom portion having an internal, axially inwardly directed conical part; said container having an inside wall face and an outside wall face, with the inside wall face of said tubular body portion adjacent said bottom portion and extending onto said bottom portion having circumferentially spaced, radially extending, continuous alterations in wall thickness with a regularly undulating cross-section across the entire circumference of the inside wall face which is progressive and gradual.
It is highly desirable to develop controllable means for improving the strength of the bottom structure producing the above-described container or the like from its corresponding parison, so as to distribute the wall thickness of the latter to thicken predetermined bottom portions thereof and to strenghen them, as by orientation.
It is therefore a principal object of the present invention to provide a method for preparing blow molded plastic containers from a plastic preform having a reinforced bottom structure controllably thickened in predetermined regions and capable of being oriented.
It is a further object of the present invention to provide a method as aforesaid which is simple, convenient and expeditious and is readily suitable for ue on a commercial scale wherein the bottom structure is characterized by thickened bottom regions without detrimental loss of properties in the remainder of the container.
Further objects and advantages of the present invention will appear hereinbelow.